1. Field of the Invention
The present invention relates to electronic signal processing and, particularly, to a method for determining skew or phase difference between digital signals.
2. Description of the Prior Art
Frequency and timing resolution in digital electronic systems are often limited by the rate at which the signals of interest can be sampled and converted into usable digital information. The uniform sampling theorem holds that a signal must be sampled at a frequency that is at least twice the maximum frequency of the components it is desired to resolve. Although this theorem defines the theoretical minimum sampling rate, errors introduced by other factors result in a practical limit which is somewhat higher than the theoretical limit.
Sampling rates have increased significantly in recent years and are projected to continue increasing. One error which is becoming more of a factor with the ever increasing sampling rates is known as channel-to-channel skew. Channel-to-channel skew is the phase difference with which the sampling pulses arrive at the system sampling points after propagation along different paths from an internal signal source. The amount of error introduced by channel-to-channel skew is dependent upon the magnitude of the skew in relation to the period of the sampling pulses. A 3 ns skew between sampling pulses having a period of 20 ns is, for example, relatively insignificant. In today's modern logic analyzer technology, however, sampling rates routinely exceed 200 MHz and are approaching 1000 MHz. At these rates, a 3 ns skew may equal or exceed the system's basic sample period. Measurement errors are a direct result.
Channel-to-channel skew is a product of a number of factors. As previously mentioned, the sampling pulses are typically generated by a common source and are propagated to the sampling points along different transmission paths. In theory, these pulses should propagate at the speed of light with imperceptible delay. However, physical and electrical properties of materials constrain this limit to some degree. Small variances in the lengths of cables or plating paths on circuit boards introduce different delays. Statistical tolerance variations in electronic components are another cause of delay.
To compound matters, signal transmission delays are not static and susceptible to complete correction through precision manufacturing. Temperature variations and component aging introduce slowly varying delays which drift over time.
From the above, it can be seen that channel-to-channel skew cannot be easily characterized. Although delays can be reduced through the use of low tolerance components and precise manufacturing techniques, the added production expense is prohibitive. This is particularly true in the case of sophisticated 64 input channel logic analyzers. However, the inherent skew of an instrument varies relatively slowly over time. Thus, a software compensation approach appears to be a more cost effective and accurate long term solution to the problem. If, prior to a test, channel-to-channel skew can be measured, a software compensation routine can be used to calibrate the system and reduce the errors attributable to skew.
It is, therefore, desirable to develop a method for measuring channel-to-channel skew in a digital electronic system. In order to be cost efficient, this method should be capable of being easily implemented by the software and hardware already present within the system. Also, the method should be capable of resolving skew to an accuracy of at least one order of a magnitude less than the period of the sampling pulses. Further, the method must be fast and repeatable.